btrfs: drop the transaction handle from the prealloc helpers

btrfs: drop the transaction handle from the prealloc helpers

The v1 space cache created its inode during the transaction commit, and
btrfs_prealloc_file_range_trans() existed so that preallocation could
reuse the open handle. It was the only caller passing a transaction, so
__btrfs_prealloc_file_range() and insert_prealloc_file_extent() now
always start their own. Fold the wrapper into
btrfs_prealloc_file_range() and drop the parameter.

Assisted-by: Claude:claude-fable-5-1
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Signed-off-by: David Sterba <dsterba@suse.com>